Silicon nitride, with its high temperature strength, tribological properties, and chemical resistance is finding increasing interest and use in critical engineered applications, such as engine co ponentry, industrial pump components, cutting tools, and in refractory and electrical applications
Reaction bonded silicon nitride is commonly prepared by reacting and nitriding silicon (either as a powder or as a formed article also known as a green body) with nitrogen by exposing the silicon to a nitrogen-containing atmosphere at temperatures of 1000.degree. C. to about 1450.degree. C. for times sufficient to produce the silicon nitride. For parts which require enhanced strengths, sintering and/or densification of the reaction bonded silicon nitride may be performed.
For improving the sintering process, various sintering aids have been used in the past. Sintering aids are generally added to the silicon powder before compacting into the green body for nitriding. The following prior art patents describe prior art attempts to enhance sintering.
U.S. Pat. No. 4,285,895 to Mangels et al. teaches that densified reaction bonded silicon nitride articles can be made by the incorporation of a densification aid into the reaction bonded silicon nitride article, surrounding the article with a packing powder of silicon nitride and densification aid and subjecting the article and powder mixture to a temperature above 1700.degree. C. with a nitrogen atmosphere of sufficient pressure to prevent volatilization of the silicon nitride for a time sufficient to permit sintering of the reaction bonded silicon nitride articles.
Several methods for introducing the densification aid into the reaction bonded silicon nitride article are disclosed in the Mangels et al. patent. These include the impregnation of the densification aid into the reaction bonded silicon nitride article; incorporation of the densification aid into the packing powder and relying upon diffusion of the densification aid into the article at the sintering temperature; and incorporation of the densification aid into the silicon powder mix prior to nitriding. The densification aids cited are magnesium oxide, yttrium oxide, cerium oxide, and zirconium oxide. The patent also teaches that the nitrogen pressure at the sintering temperature may be in the range of 250 to 1500 psi.
U.S. Pat. No. 4,235,857, to Mangels teaches that silicon can be nitrided using a demand nitriding cycle over the temperature range of 900.degree. C. to 1420.degree. C. in an atmosphere consisting of a mixture of nitrogen, hydrogen and helium. The chemical composition of the nitriding gas is constantly changing during the nitridation of the silicon article, with the chemical activity of the nitrogen decreasing (partial pressure of nitrogen in the furnace decreases) as the temperature increases. The examples cited by Mangels have nitriding times of from 130 to 175 hours.
U.S. Pat. No. 4,351,787 to Martinengo et al. teaches that sintered silicon nitride articles can be prepared by forming a silicon powder mixture containing one or more sintering additives into a compact, the additives being present in the powder in an amount such as to ensure an additive content of from 0.5 to 20 wt % in the silicon nitride compact; heating the compact under a nitrogen gas blanket at a temperature not exceeding 1500.degree. C. to convert the silicon into reaction bonded silicon nitride; and sintering the reaction bonded silicon nitride compact by heating in a nitrogen gas atmosphere at a temperature of at least 1500.degree. C. Furthermore, it is taught that the silicon powder size is 0.1 to 44 microns in size and of high purity or containing only very small amounts of nitriding catalysts. The patent teaches that any conventional sintering additive may be used. Best results are said to be achieved by using MgO, and especially in combination with Y.sub.2 O.sub.3. Other preferred additives mentioned in the patent are MgO, Y.sub.2 O.sub.3, CeO.sub. 2, ZrO.sub.2, BeO, Mg.sub.3 N.sub.2, and AlN. Other examples of additives are given as Mg.sub.2 Si, MgAl.sub.2 O.sub.4, and rare earth additions such as La.sub.2 O.sub.3. Also iron can be used with advantage, usually in mixture with conventional additives such as MgO, Y.sub.2 O.sub.3, and CeO.sub.2.
Reference is made in the Martinengo et al. patent to the use of a conventional two-stage nitriding procedure at individual heating settings of 1350.degree. C. and 1450.degree. C., the two nitriding times generally being 16 to 32 hours at 1350.degree. C. and 4 hours at 1450.degree. C. in a nitrogen atmosphere. The examples given use an atmosphere of flowing nitrogen for a period of 100 hours, the temperature being gradually brought from 1100.degree. C. to 1390.degree. C. with periods of stay at intermediate temperatures. Furthermore, samples were sintered at 1700.degree. to 1800.degree. C. for times of 1 to 3 hours, using a protective powder consisting of a combination of Si.sub.3 N.sub.4, BN, and various sintering aids.
As a final example of sintered reaction bonded silicon nitride practice, reference is made to U.S. Pat. No. 4,443,394 to Ezis which teaches a method for making a fully densified silicon nitride body. The basic principle taught is that silicon nitride will not sinter by itself, but requires a liquid phase at the sintering temperature. Ezis found that, by having an yttrium oxynitride and alumino-silicate liquid present at sintering temperatures of 1650.degree. C.-1750.degree. C., the need for an over pressure of nitrogen and cover or packing powder during sintering could be eliminated in order to densify the silicon nitride.
The Ezis patent teaches that, by (1) forming a nitridable mixture of: silicon powder, SiO.sub.2 (carried with the Si metal , Y.sub.2 O.sub.3 and Al.sub.2 O.sub.3 ; (2) nitriding the mixture to form a reaction bonded silicon nitride, with consequent formation of a Y.sub.10 Si.sub.6 O.sub.24 N.sub.2 phase, and an alumino-silicate which resides on the silicon nitride grains; and then (3) sintering in the 1650.degree. C. to 1750.degree. C. temperature range for 5-12 hours, a substantially fully densified silicon nitride is produced which exhibits a 4-point bending strength of 100,000 psi.
The Ezis patent further teaches the need for a long ball milling time of 48 hours, preferably dry, a nitridation cycle time of 200 hours, and sintering times of 5-12 hours. Total processing time including the milling can be estimated from the preferred embodiment as approximately 260 hours. This processing time is rather extensive and limits its commercial feasibility.
Many of the densification aids mentioned above or others used in the past are relatively expensive, are not always readily available, and require relatively high sintering temperatures for effectiveness.
In the process of arriving at an improved method for making a body of sintered silicon nitride which is (1) more commercially viable than prior art methods, (2) uses materials which are readily available, and (3) sinters at lower temperatures than typical prior art methods, a new non-glassy phase was found in certain sintered silicon nitride compositions.